1. Field of the Invention
The present invention relates to a semiconductor laser diode and a method for manufacturing the same. More particularly, the invention relates to a semiconductor laser diode and a method for manufacturing the same in which an internal current injection groove is formed by a selective growth.
2. Description of the Related Art
Generally, a laser diode has a refractive index waveguide type structure to obtain a stable, single mode, low threshold voltage for driving and high quantum efficiency. Such laser diodes of the refractive index waveguide type usually have an internal current confinement layer positioned at the upper side of an active layer or the lower side of the active layer in accordance with the conductive type of a substrate, so as to effectively confine the current.
A method for making a conventional laser diode having the aforementioned current confinement layer involves first growing the current confinement layer and then selectively etching the same to form a current injection groove. FIG. 1 shows a sectional view of a conventional semiconductor laser diode in which a current injection groove is formed by a selective etching.
The manufacturing process of the conventional semiconductor laser diode will now be described in conjunction with FIG. 1.
First, over a substrate 1, a first clad layer 2, an active layer 3, a second clad layer 4, a first semiconductor layer 5, a current confinement layer 6 and a second semiconductor layer 7 are formed in that order. The second semiconductor layer 7 is then selectively removed with an etching solution. A predetermined portion of the current confinement layer 6 is then slope-etched to form a current injection groove.
Subsequently, a third clad layer 8 and a cap layer 9 are formed in that order and then electrodes 10, 11 are formed over the upper surface of cap layer 9 and the bottom surface of substrate 1, respectively.
In such a semiconductor laser diode, the clad layer 4 is directly exposed without the first semiconductor 5 since it is impossible to merely etch the current confinement layer 6 selectively. In this connection, a semiconductor laser diode has been proposed in which growth layers are formed by a metal organic chemical vapour deposition and then a current injection groove is formed by a selective etching, as above mentioned, by Japanese patent publication No. 63-49396 and European patent No. 0,132,081.
The proposed method selectively etches a GaAs, serving as the current confinement layer 6, and AlGaAs, serving as the second clad layer 4, with NH.sub.4 OH:H.sub.2 O as the etching solution, to form the current injection groove. However, a portion of AlGaAs serving as the second clad layer 4 corresponding to the current injection groove is exposed and the exposed AlGaAs is oxidized, thereby significantly affecting the characteristic of the semiconductor laser diode.
European Patent No. 0,142,845 has used Al.sub.0.7 Ga.sub.0.3 As for the material of the current confinement layer 6, so that AlGaAs serving as the second clad layer 4 is not oxidized. In the method of this patent, with reference again to FIG. 1, first, over an n-type GaAs substrate 1, an n-type Al.sub.0.45 Ga.sub.0.55 As clad layer 2, an undoped Al.sub.0.45 Ga.sub.0.55 As active layer 3, a p-type Al.sub.0.45 GaAs 4 having a thickness of 0.1 .mu.m, a p-type GaAs oxidation prevention film 5 having a thickness of 0.005 .mu.m, an n-type Al.sub.0.7 Ga.sub.0.3 As current confinement layer 6 having a thickness of 0.8 .mu.m, and an n-type GaAs oxidation prevention film 7 having a thickness of 0.005 .mu.m are grown in this order, using a molecular beam epitaxy (MBE) method.
Subsequently, a photoresist is subjected to a photolithography process to form a stripe groove pattern having a width of 20 .mu.m (not shown) and then the n-type GaAs oxidation prevention film 7 is etched with H.sub.2 O.sub.2 :NH.sub.4 OH--5:1 serving as an etching solution using the stripe groove pattern as a mask.
Subsequently, if the n-type Al.sub.0.7 Ga.sub.0.3 As current confinement layer 6 is etched by an HF solution, the surface of the p-type GaAs oxidation prevention film is exposed and the etching is stopped. Thereafter, a p-type Al.sub.0.45 Ga.sub.0.55 As 8 and a p-type GaAs 9 are grown in that order by an MBE method and then electrodes 10,11 are formed.
If a current, that is, carriers are injected to the semiconductor laser diode shown in FIG. 1 through the electrodes 10, 11, the internal current confinement layer 6 confines the current effectively. Accordingly, it is possible to obtain the semiconductor laser diode having a low threshold current.
According to the conventional method for forming the current injection groove, however, it is very difficult to maintain its uniformity on a wafer having a large area for mass production. That is, since the current confinement layer is etched to form the current injection groove, it is impossible to uniformly form growth layers with the MBE method or metal organic chemical vapour deposition method on a wafer having a large area.
Also, according to Japanes Patent Publication No. 63-49396 and European Patent No. 0,132,081, the AlGaAs layer is exposed and oxidized after the etching for forming the current injection groove. Accordingly, the quality of a layer to be formed on the AlGaAs may be reduced, thereby deteriorating the reliability of the semiconductor laser diode.
As apparent from the above description, European patent 0,142,845 makes it possible to solve the above problems. However, the technique has disadvantages in that the etching process should be executed twice to form the current injection groove, it is difficult to control the etching in a lateral direction, and the side portion of the current injection groove is oxidized after the etching. Also, since Al.sub.0.7 Ga.sub.0.3 As is used as the material of the current confinement layer, the technique has another disadvantage where n-type oxidation prevention film should be essentially grown on the Al.sub.0.7 Ga.sub.0.3 As layer.